I've only been on one MX track, the Huron Valley Night Hawks track over by Irish Hills. It's a private track where they have 3-4 open rides/year. The rest of the year they have like 1 ride/week for members only. It's a pretty sweet track, good for novices and more advanced riders, and it's pretty long.
I've been to the track over in Milan, but haven't ridden it yet. It's kind of intimidating to me(beginner). Couple of big jumps and like a 70-100' table top.